An excessive driving force when the vehicle is traveling on a slippery road such as a snow-laden road tends to cause a slip of the driving wheels. This results in a reduction of gripping force of the tires, acceleration characteristics and drivability. To deal with such a case, a conventional driving force control apparatus or a so-called traction control apparatus has been developed and practically used.
In the traction control apparatus, a slip of the driving wheels is detected (slip detection method will be described later) and, when the slip is considerable, the output of the engine is forcibly and rapidly suppressed to reduce the driving force of the driving wheels (driving force reduction method will be described later). As described above, the slip is suppressed by forcible reduction of the driving force to improve the starting and acceleration characteristics on a slippery road such as a snow-laden road.
When the traction control apparatus is applied to a two-wheel driving (2 WD) vehicle, a slip amount DVS has been detected as follows. For a front wheel driving vehicle, for example, first a target driving wheel speed V.sub.OT (this is in one to one relation to a vehicle body speed V.sub.B) is calculated based on the rear wheel (driven wheels) speed. Then, speeds of the right and left front wheels (driving wheels) are averaged to determine an average driving wheel speed V.sub.FX. The target driving wheel speed V.sub.OT is then subtracted from the average driving wheel speed V.sub.FX to detect the slip amount.
Further, a control device for reducing the engine output (driving force) to suppress a slip includes the following:
(1) Throttle control (throttle valve of the intake system is closed)
(2) Ignition timing retard control (ignition timing of the ignition plug is retarded)
(3) Fuel control (fuel cutting or fuel injection amount control)
(4) Cylinder number control (operation of a predetermined number among plural cylinders is suspended).
Most of the conventional traction control apparatuses use a combination of the throttle control and the ignition timing retard control. According to the throttle control, the engine output can be controlled smoothly and over a wide range. In an abrupt start or when the road surface condition suddenly changes from a dry road to a frozen road, an abrupt slip occurs. The throttle control is not able to control such an abrupt slip. When an abrupt and excessive slip occurs, the ignition timing retard control is temporarily operated to suppress the abrupt slip with a good response.
In a traction control apparatus using a combination of the throttle control and the ignition timing retard control, setting (control begins to reduce the driving force) and resetting (control ends to restore the driving force) of the both controls are performed as follows.
First, setting and resetting conditions of the ignition timing retard control will be described. In the ignition timing retard control, setting and resetting are made in dependence on the conditions of the slip amount DVS and a slip rate GDVS which is obtained by differentiation of the slip amount DVS. That is, the control is set when both the slip amount DVS and the slip rate GDVS are greater than predetermined values. It is reset when one of the slip amount and the slip rate GDVS is decreased to some extent, and the other is considerably reduced. In practice, these are made as follows, wherein G indicates the gravitational acceleration.
The ignition timing retard control is set when both conditions (1-1) and (1-2) shown below are simultaneously met: EQU Slip amount DVS.gtoreq.2 km/h! (1-1) EQU Slip rate GDVS&gt;0.6 G. (1-2)
The setting is made when the slip amount DVS and the slip rate GDVS are both greater than the predetermined values.
The ignition timing retard control is reset when both conditions (2-1) and (2-2), or (3-1) and (3-2), shown below are simultaneously met: EQU -0.5 G.ltoreq.GDVS.ltoreq.0 G; (2-1)
and EQU DVS&lt;6 km/h! (2-2)
or EQU GDVS&lt;-0.5 G; (3-1)
and EQU DVS&lt;18 km/h!. (3-2)
The above (2-1) and (2-2) are conditions where the slip rate GDVS is decreased to some extent and the slip amount DVS is considerably decreased. The above (3-1) and (3-2) are conditions where the slip amount DVS is decreased to some extent and the slip rate GDVS is considerably decreased.
Next, setting and resetting conditions of the throttle control will be described. In the throttle control, the throttle control is set when the slip amount DVS is greater than a preset value. It is reset when the slip amount DVS is smaller than a preset value. The steps are as follows.
Details will be described in the embodiments of the present invention. To achieve slip suppression control by the throttle control, first a reference driving torque T.sub.B to maintain a vehicle body speed V.sub.B is calculated, and feedback correction torque T.sub.F which is a cause of the occurrence of a slip is calculated based on the slip amount DVS. The feedback correction torque is T.sub.F is determined by PID (proportional, integration, differentiation) calculating the slip amount DVS. Therefore, by subtracting the feedback correction torque T.sub.F from the engine output torque, the slip amount DVS can be reduced. Then, the feedback correction torque T.sub.F which is a cause of slip occurrence is subtracted from the reference driving torque T.sub.B to determine the target driving torque T.sub.O. This thereby reduces the engine output torque to the target driving torque T.sub.O. By reducing the engine output to the target driving torque T.sub.O, that is, by suppressing an excessive torque which is a cause of slip occurrence, slip occurrence is suppressed. As the torque reduction device in this case, the previously described fuel control or cylinder number control can be used instead of the throttle control.
In a vehicle using the traction control apparatus, when traveling, for example, on a split road (a road surface where the frictional coefficient of the road surface is different between the right and left wheel, that is, where one side road surface is the ground and the other side is snow-laden or frozen), a high vehicle body vibration may occur. The vehicle body vibration can be broadly divided into the following two causes.
Initially, a first cause of vehicle body vibration will be described.
When a driving force control apparatus (traction control apparatus) in which the target driving wheel speed V.sub.OT is decreased from an average driving wheel speed V.sub.OT to determine the slip amount DVS, and setting and resetting of the ignition timing retard control are made according to the above conditions (1-1), (1-2), (2-1), (2-2), (3-1) and (3-2) is used on a 2 WD vehicle, and when the vehicle travels on a split road (a road surface where the frictional coefficient of the road surface contacting the driving wheels differs between the right and left wheel), a high vehicle body vibration may occur.
The cause of occurrence of the high vehicle body vibration will be described with reference to FIG. 6. When the vehicle travels on a split road, a single-wheel slip condition may occur where only one of the right and left driving wheels slips. In such a single-wheel slip condition, for example, since the left driving wheel slips whereas the right driving wheel does not slip, the average driving wheel speed V.sub.FX abruptly increases. Thus, the on-board computer outputs a retard command to make the ignition timing retard control. When the ignition timing retard control is made, and the target driving wheel speed V.sub.OT is made, the ignition timing retard control is ended. If the road surface is still in a split condition even after the ignition timing retard control ends, a single wheel slip occurs again. Further, the average driving wheel speed V.sub.FX again abruptly increases to make the ignition timing retard control. Then, the ignition timing retard control is ended. Such operation is repeated to output a plurality (e.g. 7) of retard commands in a short time (e.g. 1 second), and the ignition timing retard control is carried out in a hunting condition.
When the ignition timing retard control is carried out in a hunting condition as described above, a hunting phenomenon occurs where speed fluctuation of the driving wheels is vibrating. The vibration of the driving wheels is transmitted through the transmission system to the engine. In particular in a manual transmission using a gear mechanism, vibration of the driving wheels is liable to be transmitted to the engine as compared to an automatic transmission using a hydromatic torque converter. When the period of the vibration transmitted to the engine is close to the natural frequency of the engine and both frequencies are resonant with each other, the engine strongly vibrates, resulting in a high vehicle body vibration.
Since the engine is mounted on the chassis through an elastic material such as rubber, it is liable to vibrate. Further, since it is heavy in weight, only one vibration results in a high vehicle vibration occurring.
Next, a second cause of vehicle body vibration will be described.
When the vehicle travels on a split road or the like, one driving wheel contacting a low-friction road surface such as a snow-laden road may lose resistance, and the other driving wheel contacting high-friction, normal road surface may have a high resistance. In such a condition, by the function of the differential gear, in an extreme case, only one driving wheel with no resistance is rotated, and the other driving wheel with a resistance is not supplied with the driving force. That is, a strong driving force is transmitted to only one driving wheel. When a strong driving force is transmitted to only one driving wheel, a torsional force is generated and the speed of one driving wheel is rapidly increased. Further, vibration due to the torsional force and the rapid increase in the driving wheel speed acts upon the spring resonant system such as suspension, resulting in an even higher vibration. The vibration and torsional force are transmitted to the driving wheels
Further, in a vehicle using a manual transmission with a gear mechanism, the above-described torsional force and the vibration is amplified by the spring resonant system and are transmitted to the engine to vibrate the engine. Moreover, vibration of the engine is returned to the driving wheels through the transmission, propeller shaft, and differential gear. Ultimately, the torsional force and vibration of the spring resonant system are directly transmitted to the driving wheels, and the engine vibration caused by the vibration of the spring resonant system and the like are returned and transmitted. As a result, the rotational speed of the driving wheels vibratingly fluctuates.
When the rotational speed of the driving wheels vibratingly fluctuates, the value of the slip amount DVS also vibratingly fluctuates. This is because the vibratingly fluctuating average driving wheel speed is subtracted from the target driving wheel speed (corresponding to the vehicle body speed) to obtain the slip amount DVS.
When the value of the slip amount DVS vibratingly fluctuates, the value of the feedback correction torque T.sub.F, which is obtained by PID calculation of the slip amount DVS, largely vibratingly fluctuates. Of the components of the feedback correction torque T.sub.F, particularly the value of a component obtained by differentiation largely fluctuates in positive and negative directions.
When the value of the feedback correction torque T.sub.F largely vibratingly fluctuates, the value of the target driving torque T.sub.O, obtained by subtracting the feedback correction torque T.sub.F from the reference driving torque T.sub.B, also vibratingly fluctuates.
In the traction control apparatus, the output torque is controlled so that the engine output is equal to the target driving torque T.sub.O. However, if the target driving torque vibratingly fluctuates, the output torque also vibratingly fluctuates. This results in a high vehicle body vibration.
When the engine output (torque) is attempted to be reduced for slip suppression during traveling on a split road, fluctuation of the driving wheel speed becomes vibrating. A technology which prevents vibration by adjusting the control gain to reduce the proportion of torque reduction is also disclosed in Japanese Patent Laid-open Publication 2-252930. However, this technology has the following problems.
(1) When traveling on a split road is detected, since the control gain is reduced not only in the torque increasing direction but also in the torque reducing direction, the response of the objective slip reduction control also tends to be decreased.
(2) Further, since the above control is made only when traveling on a split road, it cannot appropriately deal with a vibration actually occurring due to another cause at a position other than a split road.